Molecular formula from high-resolution mass spectrometry

High-resolution mass spectrometry involves the use of a detector that can measure tw/z values to four decimal places. This technique allows for the determination of the molecular formula of an unknown compound. In order to analyze the data obtained from high-resolution mass spectrometry, we must first review some background information. Specifically, we must discuss why atomic masses are not whole numbers (despite the fact that we have treated them as such until now). 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

Camphor, a saturated monoketone from the Asian camphor tree, is used among other things as a moth repellent and as a constituent of embalming fluid. If camphor has M+ = 152.1201 by high-resolution mass spectrometry, what is its molecular formula How many rings does camphor have ... 

B Nicotine is a diamino compound isolated from dried tobacco leaves. Nicotine has two rings and M + = 162.1157 by high-resolution mass spectrometry. Give a molecular formula for nicotine, and calculate the number of double bonds. 

The following is a procedure recommended for elucidating the structure of complex organic molecules. It uses a combination of different NMR and other spectroscopic techniques. It assumes that the molecular formula has been deduced from elemental analysis or high-resolution mass spectrometry. Computer-based automated or interactive versions of similar approaches have also been devised for structural elucidation of complex natural products, such as SESAMI (systematic elucidation of structures by using artificial machine intelligence), but there is no substitute for the hard work, experience, and intuition of the chemist. 

A unique molecular formula (or fragment formula) can often be derived from a sufficiently accurate mass measurement alone (high-resolution mass spectrometry). This is possible because the nuclide masses are not integers (see Table 2.2). For example, we can distinguish at a unit mass of 28 among CO, N2, CH2N, and C2H4. 

Mass spectrometry (MS) provides the molecular weight and valuable information about the molecular formula, using a very small sample. High-resolution mass spectrometry (HRMS) can provide an accurate molecular formula, even for an impure sample. The mass spectrum also provides structural information that can confirm a structure derived from NMR and IR spectroscopy. 

Identification and structural characterization of an unknown class of substrates for FAAH. (A) By high-resolution mass spectrometry, the high-accuracy mass measurements of a compound of this class gives an exact mass of 446.3310 that corresponds to a molecular formula of C24H48N04S. (B) By MS/MS analysis, the structure of this compound is assigned as the C24 0 fatty acyl amide of taurine (NAT). Reproduced from Saghatelian A., Trauger S.A., Want E.J., Hawkins E.G., Siuzdak G. and Cravatt B.F., Biochemistry, 43,14332-14339, 2004, with permission. 

Some of the most difficult structural problems in the indole alkaloid field are associated with the bisindole alkaloids of the vobtusine type. Since 1955, vobtusine has been isolated on numerous occasions, often in large quantities, from the Apocynaceae species Callichilia, Conopharyngia, Rejoua, and Voacanga A correct molecular formula could only be determined by high-resolution mass spectrometry. In 1966 a partial structure was proposed for the alkaloid and later in the same year a complete structure was put forward. An unambiguous structural proof is, however, still lacking. The difficulty arises from the complete resistance of the alkaloid and its derivatives to cleavage, in contrast, for example, to the dimers of the voacamine and vinblastine types. Non-cleavable dimers occur also in calabash-curare but in these cases chemical correlation with cleavable alkaloids has been possible (see Section 2, p. 209). To date no bisindole alkaloid related to vobtusine has been found which can be split into monomeric units. 

Dehydrocancentrine-B, a cherry-red alkaloid isolated from the same source, had the same functional groups as cancentrine (NMR, IR). However, its IR spectrum indicated the presence of an additional double bond in agreement with the molecular formula (C36H32N2O7) obtained by high-resolution mass spectrometry (HRMS). The mass spectrum was very similar to that of cancentrine with the exception that ions from the cularine half of the molecule appeared two mass units lower. Thus there were ions at m/e 361 (C21H15NO5) and m/e 348 (C20H14NO5) arising from fissions a + b and a -l- c, respectively (Scheme 1). This indicated that the extra double bond was in the cularine part of the molecule and must be located at the only available position, namely, C-31—C-32 (12). The NMR spectrum supported the location of the double bond in this position by the presence of a fourth AB system one half of which was visible at S 6.25 (Jab = 7.0 Hz). The location of the substituents and the relative stereochemistry of the alkaloid were shown to be identical with those... 

Fraction 6-5 was repeatedly purified by recrystallization from methanol and ethyl acetate to afford compound (10) as needle crystals, mp 275 280°C (dec.). Compound (10) showed no optical rotation and was almost insoluble in most organic solvents. The mass spectrum displayed the molecular ion at m/z 410.1013 by high resolution mass spectrometry, in agreement with the formula C22H1808. The... 

The stracture of haplophytine was established 21 years after its first isolation and original formula assignment. The correct molecular stracture was determined by high-resolution mass spectrometry, while the full structure was realized by extensive chemical degradation, spectroscopy, and X-ray crystallographic smdies from the groups of Cava, Yates, and Zacharias [76, 77]. 

Mass spectrometry (MS) detects the molecular mass of a compound with a precision of 10 " mass units. Owing to the isotope mass defect of elements the molecular formula of a terpene can be determined by high-resolution mass spectrometry of the molecular ion. For example, the molecular formula C17//22O4 of acanthifolin from Senecio acanthifolius (Asteraceae) is calculated from the molecular mass of 290.1525 determined by high-resolution mass spectrometry of the molecular ion. Additionally, partial structures of molecules can be derived from the masses of the ions arising from fragmentation of the molecular ion and detected in the mass spectrum. 

In an initial study of the alkaloids of Sceletium joubertii, Arndt and Kruger (31) isolated three new alkaloids. The essential features of the structures of these noncrystalline bases were evident from their spectral properties. The molecular formula of joubertiamine as C16H21NO2 was established by high-resolution mass spectrometry, and fragment ions at m/e 58 and m/e 78 indicated that the alkaloid contains an A -dimethylamino-ethyl side chain. A carbonyl absorption in the IR at 1680 cm" and the UV spectrum were both in accord with the presence of an a,jS-unsaturated carbonyl group. A bathochromic shift of the absorption maxima in the UV... 

Tortuosamine, a noncrystalline, optically active base, was isolated together with the partial racemate of the A4 alkaloid from S. tortuosum by Wiechers and co-workers (39). The molecular formula C20H26N2O2 was established by high-resolution mass spectrometry, which also provided... 

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